What is the processor you are looking at?
It should be quite simple with a Atmega chip with super small QFN package with minimum addition of crystal, a LED, a micro USB connector, a 3.3V LDO if necessary, TP4056 battery charger.
If you are planning to use a Lithium battery (max voltage 4.2V) you need to run your MCU at 3.3V.
What do you mean by captive?
What specific application you are looking at?
If there is a specific application requirement for such a setup then its definitely marketable. Arduino are used more because of the easy coding and the pluggable shields, which we wont be able to support due to small size and different footprint.